Selective catalytic reduction (“SCR”) exhaust aftertreatment systems are an important technology for reducing NOx emissions from internal combustion engines such as diesel engines. SCR systems generally include a source of reductant such as a urea solution, a pump unit for pressurizing the urea solution, a metering unit for providing a controlled amount or rate of urea solution to an SCR catalyst, and an injector which provides urea solution to a urea decomposition region of an exhaust flowpath located upstream from an SCR catalyst. Many SCR systems also utilize pressurized gas to assist the flow of urea solution to the injector.
While providing important reductions in NOx emissions, SCR systems suffer from a number of shortcomings and problems. The supply of the urea solution under pressure can result in high pressure oscillations after long hours of continuous operation of the system. High pressure oscillations adversely affect system performance and deteriorate dosing accuracy. In addition, high pressure oscillations stress system components, in some cases beyond design limits, which creates reliability and warranty concerns. There is a long felt need for advancements mitigating these and other shortcomings associated with SCR systems utilizing pressurized dosing of a reductant such as a urea solution.